The present assignee has developed a printable LED light sheet where microscopic inorganic LED dies, having a top electrode and a bottom electrode, are printed as an ink on a conductive layer on a thin substrate. Such LEDs are called vertical LEDs. The ink comprises the pre-formed LED dies uniformly infused in a solvent. After the ink is cured, the bottom electrodes of the LEDs make electrical contact to the conductive layer. A dielectric layer is then deposited between the LEDs, and another conductive layer is printed to make electrical contact to the top electrodes of the LEDs to connect the LEDs in parallel. A suitable voltage is applied to the two conductive layers to illuminate the LEDs. To allow light to escape, one or both of the conductive layers is transparent. Indium tin oxide (ITO) or sintered silver nano-wires may be used for the transparent conductive layer. Other conductive oxides may also be used. Such a technique is described in the assignee's U.S. Pat. No. 9,343,593, entitled, Printable Composition of a Liquid or Gel Suspension of Diodes, and related patents, incorporated herein by reference.
A simplified example of a single printable LED is shown in FIG. 1. The LED 10 has a bottom electrode 12, which is typically a reflective metal, and a top electrode 14. The semiconductor portion 16 may comprise a conventional GaN n-type layer and p-type layer sandwiching a GaN active layer. In one embodiment, the GaN-based LED emits blue light. The color emission is based on the material composition. In other embodiments, green and red LEDs may be printed. Light 18 is emitted from the top if the bottom electrode 12 is opaque.
The LED 10 is formed to have a relatively thin and tall top electrode 14 so that the LEDs orient themselves correctly on a substrate when printed as a liquid ink.
The assignee has also previously invented an “active LED”, as shown in FIG. 2, where a vertical LED 20 and a vertical transistor 22 are bonded together or formed on the same substrate. The active LED 24 has a top electrode 26, a bottom electrode 28, and a transistor control electrode 30. The transistor 22 may be a FET or a bipolar transistor. Such an active LED is described in the assignee's U.S. Pat. Nos. 9,661,716; 9,572,222; and 10,201,051, all of which are incorporated by reference.
FIG. 3 is a schematic diagram of the active LED 24 of FIG. 2. The transistor 22 is labeled a pMOS transistor, but it may also be an nMOS transistor.
The assignee also has various patents describing forming a full color display using the LED 10 or the active LED 24 in pixel locations in a single layer. In one embodiment, the LEDs all emit blue light, and red and green phosphors are used to create the red, green, and blue pixels for a full color display. In other embodiments, different semiconductor material compositions of the LEDs are used to create red, green, and blue emitting LEDs, and various techniques are used to energize the RGB LEDs to display images.
Since the RGB LEDs are all in the same plane on a substrate, it is difficult to form and separately energize the various RGB pixels. Forming the RGB pixels in the same plane requires the red pixels to be relatively spread out to make room for the green and blue pixels. The green and blue pixels must also be similarly spread out. Further, the column and row lines must have a high density for selectively energizing the RGB pixels. Further, if different materials are used to form the RGB LEDs, they will need different driving currents due to their different efficiencies, which complicates the drivers. If phosphors are used, the phosphors are relatively expensive, may have significant persistence times, and are difficult to precisely align with the pixels.
One example of a single-plane, addressable RGB display is described in the assignee's U.S. Pat. No. 9,368,549, incorporated by reference, and shown in FIG. 4. On a substrate 34 are formed conductive column lines 36. A reflective (e.g., white) hydrophobic mesh 38 is printed that creates a 2-dimensional grid of pixel locations. An LED ink is then printed and cured to print the LEDs 10 in each pixel location, where the bottom electrodes of the LEDs 10 electrically contact the associated column lines 36. There may be a random number of LEDs 10 in each pixel location due to the non-determinative printing process. A dielectric layer 40 is deposited between the LEDs 10. Transparent row lines 42 are then deposited to contact the top electrodes 14. It is assumed that all the LEDs 10 emit blue light. To form the red and green pixels, red and green phosphors 44 and 46 are printed over the red and green pixels. A controller 48 controls an addressable column driver 50 and an addressable row driver 52 to select individual RGB pixels to display a color image. The LEDs 10 may be simultaneously or sequentially illuminated. The brightness may be controlled by pulse width modulation or current magnitude.
Drawbacks of the single-plane display of FIG. 4 have been mentioned above.
What is needed is a technique for forming a full-color, addressable display using printed LEDs which does not have the drawbacks of the single-plane displays described above.